This invention relates to a weighing device, especially to such device wherein the weight signal produced from a loaded weigher initially oscillates (vibrates) and the period of this oscillation or vibration functionally relates to the load on the weigher.
Japanese opened patent gazette No. 55-1578 discloses a weighing device which is used as a table digital balance or a market weigher. In this device, as schematically shown in FIG. 4, a product 2 is weighed by a weigher 1 which includes a weight sensor of the electric resistance load cell or force balance type, for example, which produces an analog electric signal indicative of the weight of product 2. This signal is amplified by an amplifier 3 and converted by a time integration A/D convertor 21 into a digital signal which is displayed as a weight value by a digital display 22.
The A/D convertor 21 is operable to integrate the analog signal from the amplifier 3 for a predetermined length of time T and to divide the result of integration by the time T to obtain a digital value. The weigher 1 generally includes an elastic vibration system, such as spring, and produces a vibrating or oscillating weight indicative signal when it is initially loaded. The period of this oscillator varies with the weight of product 2 on the weigher 1. Therefore, the ratio of the above-specified integration time to the period of oscillation varies with the weight of product 2 and this results in undesirable measurement errors.
More specifically, assume now that the oscillation or vibration of the weigher 1 is sinusoidal and that the analog electric signal e output from the amplifier 3 is given as follows: EQU e=W+a sin .omega.t, EQU .omega.=2.pi./.tau.
where W is the weight of product 2, a is the amplitude of vibration and .tau. is the vibration period, as shown in FIG. 2. Then, the integrated and averaged signal from the A/D convertor 21 includes an error E as follows: EQU E=(a/T.function..sup.T sin .omega.t.multidot.dt
As is also obvious from FIG. 2, this value is zero for the value of T equal to the vibration period .tau. or its integral multiple at time points such as t1, t2 and t3 when integration starts at time t0, while it is not zero for other value of T. When the integration time T is fixed as in the prior art device, therefore, there is little chance that the error E will be zero. Accordingly, in the prior art devices, it is necessary to effect the integration in a time interval as shown by B in FIG. 3 where the vibration has substantially decayed, avoiding a vibration interval as shown by A. As a result, the weighing operation of the prior art devices is very time consuming.